Nonlinear Dynamic Of Marine Mooring Systems

Mooring systems are widely utilized in marine engineering, such as offshore oil and gas development, ocean environmental monitoring, and ocean energy development. They constitute the key elements to ensure the security and integrity of offshore structures. The nonlinear factors of mooring systems (such as stiffness nonlinearity, geometric nonlinearity, nonlinear hydrodynamic loads, nonlinear bottom boundary conditions, etc.)can affect the dynamic property, and then the safety of mooring is threatened.In this paper, the typical nonlinear problems are analyzed in mooring systems. The theory and method of nonlinear dynamic are adapted to the nonlinear dynamic of mooring systems under complex marine environment. The relationship between the non-linear dynamic response and environmental factors, structural parameters, is revealed. This paper provides the basis for the research mechanism dynamics of the nonlinear factors in mooring systems.Firstly, the dynamic equation of radial moored ship, which is solved by Harmonic Balance Method (HBM) method and numerical method, is built. The jumping phenomenon is obtained by numerical method. The motion of system is unstable in specific range of excitation frequency. Moreover, the attraction of domain of radial moored ship is discussed in the different initial positions and initial velocities. The nonlinear dynamics response of single degree of freedom segmented stiffness of the mooring system, such as articulated tower platform, is studied, and the bifurcation diagram and the largest Lyapunov exponent are obtained.Next, two-dimensional dynamic equations of mooring system are established. The dynamic response is analyzed by power spectrum, phase diagram,Poincare mapping and Lyapunov exponent. The bifurcation and chaos of mooring systems are obtained in the different exciting frequencies and different amplitudes. On this basis, the bifurcation and chaos of mooring system change little considering the nonlinear stiffness of mooring cable and combined wave-current loads, but the amplitude of mooring system and the tension of mooring will become large.Then, the dynamic model of mooring cable is established based on lumped mass method. The equilibrium state of mooring cable is obtained.Comparing with catenary method, the result is agreed well. On this basis, the endpoint of mooring cable moves with different amplitudes and different frequencies, which comes up with the result that the cable tension will become larger in pace with the increase of frequency or motion amplitude. In addition, the dynamic model of mooring system during the process of deployment and recovery is built by lumped mass method. Comparing with experiment of deployment, the result is agreed well. The analysis of recovery process proves that mooring system may meet the twine.Finally, the application of mooring system is researched in point absorber wave energy converter. Considering the nonlinear factors in PTO, this paper studies the dynamic model of point absorber wave energy converter with nonlinear damping and nonlinear stiffness. The Power Capture Width is researched in different frequencies, amplitudes and nonlinear stiffness. The model of the electromagnetic force of linear generator is established. Comparing with the simulation result of Ansys Maxwell, the result is agreed well. So the model of wave energy converter can be obtained, and the dynamic response is got. Considering the harmonic excitation, this paper studies the wave energy convector with two degrees of freedom. The power is debated in different PTO damping ratios, excitation frequency ratios, mass ratios and the tuning frequency ratios. It shows that the second mode of system can obtain more power, and ensure relatively small amplitude, which indicates that the security of mooring system does not face threat.